A placebo is a drug or treatment given with the suggestion that it will produce an effect which it would not produce in the absence of the suggestion. Because a placebo sometimes works to produce the effect suggested it is used in two ways. A placebo treatment is often given to a control group to establish a baseline against which to assess a new drug or treatment. The placebo is also used as therapy with the assumption that it may do some good, and in any event it can do no harm. However, there is some evidence that for sophisticated subjects, a placebo can have an effect the reverse of that suggested. Such data qualify both the baseline and the therapeutic use of placebos. The data can be understood in terms of a principle of attribution. Conceptual replications are proposed that are designed to produce both placebo and reverse placebo effects.